1. Field of the Invention
The present invention relates to a process and apparatus for removing unpolymerized gaseous monomers from solid olefin polymers, and more particularly and in a preferred embodiment, to a process for removing unpolymerized gaseous hydrocarbon monomers from granular, low pressure-polymerized, low density ethylene polymers, particularly "sticky polymers" such as ethylene, propylene, diene terpolymers.
2. Description of the Prior Art
It has long been known that olefins such as ethylene can be polymerized by contacting them under polymerization conditions with a catalyst comprising a transition metal compound, e.g., titanium tetrachloride and a cocatalyst or activator, e.g., an organometallic compound such a triethyl aluminum. Catalysts of this type are generally referred to as Ziegler catalysts.
Low density ethylene polymers (i.e., ethylene polymers having a density of about 0.94 g/cc and lower) have in the past been made commercially by a high pressure (i.e., at pressures of 15,000 psi and higher) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly assigned, U.S. Pat. No. 4,302,565.
This patent discloses a low pressure, gas phase process for producing low density ethylene copolymers having a wide density range of about 0.91 to about 0.94 g/cc. More recently low density ethylene copolymers have been produced having densities of about 0.86 to about 0.96 g/cc.
The resulting granular polymers produced from conventional low pressure processes usually contain gaseous unpolymerized monomers including hydrocarbon monomers. These gaseous monomers should be removed from the granular resin for safety reasons, since there is a danger of explosion if the hydrocarbon monomer concentration becomes excessive in the presence of oxygen. In addition, proper disposal of the hydrocarbon is required in order to meet environmental standards concerning hydrocarbon emissions.
The prior art teaches techniques for removing volatile unpolymerized monomers from polymers of the corresponding monomers. See for example, U.S. Pat. Nos. 4,197,399, 3,594,356 and 3,450,183.
U.S. Pat. No. 4,372,758 issued Feb. 8, 1983 to R. W. Bobst et al and which is assigned to a common assignee discloses, a degassing or purging process for removing unpolymerized gaseous monomers from solid olefin polymers. The purging process generally comprises conveying the solid polymer (e.g., in granular form) to a purge vessel and contacting the polymer in the purge vessel with a countercurrent inert gas purge stream to strip away the monomer gases which are evolved from the polymer.
Unfortunately, however, when producing certain types of ethylene polymers, such as "sticky polymers" certain problems are encountered because of the type of monomers to be removed. Although these sticky polymers can be rendered non-sticky in the reactor, see for example U.S. Pat. No. 4,994,534 issued Feb. 19, 1991, there still remains the problem of effectively removing diene monomers, e.g., ENB from the polymers during the purging process. For example, in producing ethylene, propylene, diene terpolymers, monomers such as ethylidene norbornene (ENB) remaining in the product must be substantially purged from the product due to cost and environmental considerations. However ENB has a significantly low diffusivity. If the traditional packed bed process is used, it would require an impractically long residence time or an extra large amount of purge gas. It is clear that the conventional packed bed process is not entirely suitable for ENB purging.
Thus, the versatility of the resin post-reaction treatment processes have not evolved at the pace of the gas phase reactor technology which has been extended to the production of fluidizable but non-free flowing solid polymer as disclosed in U.S. Pat. No. 4,710,538 issued Dec. 1, 1987. These non-free flowing granular resins are referred to sometimes as "sticky polymers" because of their tendency to aggregate in larger particles and to eventually form large chunks of solid polymer.
The term "sticky polymer" is defined as a polymer, which although particulate at temperatures below the sticking temperature, agglomerates at temperatures above the sticking temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to the agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the resin, it is predominantly governed by the temperature and the crystalinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline product such as very low density polyethylene (VLDPE), ethylene/propylene monomer (EPM), ethylene/propylene diene monomer (EPDM) and polypropylene (PP) copolymers usually display a larger tendency to agglomerate in larger particles.
The mechanical agitation of a fluid bed or stirred gas phase reactor is somewhat sufficient to prevent the agglomeration of sticky polymers in the vessel. However current post reactor processing and handling equipment utilized with gas phase reactors are not specifically designed to process sticky polymers. For example the conventional packed bed purging processes tend to have uneven gas distribution in the purge vessel. Purge gas seems to bypass through certain channels without contact with the majority of solids. It is submitted that this poor solid-gas contact condition in the conventional packed bed process is the primary reason that purging is at least one order of magnitude worse than theoretical prediction.
More recently, copending application Ser. No. 07/701,999 filed May 17, 1991, now abandoned and assigned to a common assignee attempts to ameliorate the prior art problems by providing a process for removing unpolymerized gaseous monomers from a solid olefin polymer by utilizing a purge vessel provided with at least one substantially vertically disposed grid plate positioned substantially transversely across said vessel. The grid plate defines openings in an amount and size sufficient to permit passage of solid olefin polymer and an inert gas through said at least one grid plate. An inert feed gas is fed to the purge vessel and through the openings in the grid plate in countercurrent contact with the polymer, the inert purge gas being utilized in an amount and at a velocity sufficient to form a fully expanded bed in the purge vessel.
Although this process overcomes many disadvantages incident to prior art techniques, it is still not the panacea since the process requires strict control of the superficial gas velocity in order to obtain expanded bed operation.
The present invention provides an improvement over the process disclosed in application Ser. No. 07/701,999, now abandoned in that the superficial gas velocity need not be precisely controlled in order to obtain expanded bed operation. The terms "expanded bed" or expanded fluidized bed as used herein means that substantially every single solid in the bed is lifted and supported by the drag force of the purge gas.
The term "packed bed" as used herein means that when the gas superficial velocity increases, the height of the resin bed increases slightly and the pressure drop of the resins increases proportionally to the gas superficial velocity. There is no sign of substantial formation of bubbles in the resin bed.
The term fluidized bed as used herein means that when the gas superficial velocity increases, the height of the resin bed and the pressure drop across the bed do not change. Bubbles travel through the whole bed and large scale bed circulation is observed.
The term expanded bed operation as used herein means that when the gas superficial velocity increases, the height of the resin bed increases significantly and the pressure drop across the resin bed increases proportionally to the gas superficial velocity. There is no sign of bubbles in the resin bed. Slight resin motion can be observed locally, but there is no large scale bed circulation.
Thus a solid bed is in a fully expanded mode when substantially every single solid in the bed is lifted and supported by the drag force of the purge gas. Therefore, a process operated in the expanded bed mode can insure that each solid in the expanded bed will be swept by the purge gas and thus offer an excellent solid-gas contact situation.